When a fault develops in a generating system, the resulting excessive load on the system caused by the fault can cause grave damage to the system as a result of overheating or other associated problems. While the problems associated with the development of faults are of concern in almost all generating systems, they may be particularly acute in the generating systems employed in aircraft having high electrical power requirements. In particular, aircraft generating systems are quite expensive in relation to their generating capacity and if damage is sustained as a result of a fault, it is commensurately more expensive to repair. Moreover, in many instances, the economical operation of aircraft depends upon a high rate of utilization of such aircraft. Consequently, excessive down time of the aircraft for repair of fault caused damage to a generating system may severely impede efficient use of such aircraft.
Consequently, such power systems must be provided with control units that are capable of detecting faults and in turn, prevent the fault condition from causing the generator to cycle on and off continuously, a phenomenon sometimes termed "doorbelling".
A particularly troublesome fault condition in generating systems is that occassioned by a short circuit to electrical ground on the supply side of the generator exciter field winding. In a typical generating system, a permanent magnet generator provides power for the control unit and to a rectifier bridge which in turn conveys power to the exciter field winding of the main generator. Typically, the control unit operates an electrically held relay which opens contacts in the exciter field supply circuit when power is removed from the control unit or when a fault detector determines that a fault condition exists. When the relay is closed, power is supplied to the exciter field winding. Conversely, when the relay is opened, power is removed from the exciter field winding.
Under normal conditions, the control unit will cause the relay to close to connect the exciter field winding into the circuit. Should a fault occur, the resulting load causes the permanent magnet generator to be extremely heavily loaded and its voltage output is accordingly greatly reduced. The permanent magnet generator will not develop sufficient voltage to power the control unit and as a consequence, it cannot continue to command the relay to maintain a closed condition and the relay opens.
When the relay opens, the heavy loading on the permanent magnet generator ceases to exist and the permanent magnet generator then provides a full voltage output. At this point, the control unit receives enough voltage to resume operation and causes the relay to close. As a consequence, the voltage at the output of the permanent magnet generator again becomes greatly reduced because of the heavy loading caused by the reconnected fault. Sufficient voltage to power the control unit is again lost and the relay will again open.
This continuous cycling will occur indefinitely until there is manual intervention.
To avoid this problem, in some prior instances, an auxiliary source of power whose voltage is independent of the output of the permanent magnet generator is utilized to provide power to the control unit. When such is used, when the control unit opens the relay in response to the detection of a fault, the relay will remain open so long as the control unit is powered by the auxiliary source of power and is unaffected by the immediate increase in output voltage at the permanent magnet generator upon disconnection of the fault therefrom.
While this approach represents a solution to doorbelling, it requires the presence of an auxiliary source of power; and such may not be readily available, or available at all in various systems.
The present invention is directed to overcoming one or more of the above problems.